tools/perf/build: Simplify the libelf logic

Ulrich Drepper and Namhyung Kim reported that the libelf logic in
config/Makefile is duplicated in part.

Remove the duplication, and also remove the now unused FLAGS_LIBELF
variable.
Reported-by: NUlrich Drepper <drepper@gmail.com>
Reported-by: NNamhyung Kim <namhyung@kernel.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Signed-off-by: NIngo Molnar <mingo@kernel.org>
Signed-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/config/Makefile

@@ -280,7 +280,6 @@ endif # NO_LIBELF
 
 ifndef NO_LIBELF
   CFLAGS += -DHAVE_LIBELF_SUPPORT
-  FLAGS_LIBELF=$(CFLAGS) $(LDFLAGS) $(EXTLIBS)
 
   ifeq ($(feature-libelf-mmap), 1)
     CFLAGS += -DHAVE_LIBELF_MMAP_SUPPORT
@@ -305,13 +304,6 @@ ifndef NO_LIBELF
   endif # NO_DWARF
 endif # NO_LIBELF
 
-ifndef NO_LIBELF
-  CFLAGS += -DHAVE_LIBELF_SUPPORT
-  ifeq ($(feature-libelf-mmap), 1)
-    CFLAGS += -DHAVE_LIBELF_MMAP_SUPPORT
-  endif
-endif # NO_LIBELF
-
 # There's only x86 (both 32 and 64) support for CFI unwind so far
 ifneq ($(ARCH),x86)
   NO_LIBUNWIND := 1